On Sun, Aug 07, 2016 at 11:33:17AM -0400, Alan Stern wrote: > On Sun, 7 Aug 2016, Lukas Wunner wrote: > > > Normally the device hierarchy is traversed bottom-up during suspend > > and top-down during resume. However ->prepare and ->complete do it > > the other way round. In the case of ->prepare, this is even documented > > in Documentation/power/devices.txt but the reason thereof is not. > > Could you explain this please? > > The purpose of ->prepare is to tell drivers that a system sleep is > beginning and accordingly they should stop registering new children. > This is necessary for the PM core to be able to traverse the entire > device tree safely; we want to avoid races where a new child is added > below a device concurrently with that device being suspended. (Or if > you want to be more precise, races in which a new child is added below > a device while the PM core is acquiring the device's lock just prior to > invoking its ->suspend callback.) > > Telling drivers to stop registering new children below a device has to > be done top-down, because if it were done bottom-up then it would be > subject to the same race described above. Doing it top-down avoids > problems; if a device registers new children while the PM core is > acquiring its lock prior to invoking ->prepare, it doesn't matter. The > new children will be handled later, right along with the existing ones. Thank you for explaining the motivation to carry out ->prepare top-down. However my problem is really that ->complete is carried out bottom-up. What's the motivation for that? Merely to mirror the behaviour of ->prepare? Would it be possible to change it to top-down? Note that re-enablement of device addition is already allowed in ->resume, which is called top-down. By the way, neither the PCI nor USB bus-level ->prepare callbacks perform any action that would stop device addition. Same for the pciehp driver (we don't even have a ->prepare callback defined for PCIe port services. So it *is* possible to hotplug PCI devices after ->prepare. Best regards, Lukas -- To unsubscribe from this list: send the line "unsubscribe linux-pci" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
